Many industries require a means for sensing the presence of volatile acidic or basic chemical compounds in a fluid (gas or liquid). The means should provide a visual indication of the presence of the compound of interest or provide an instrument for monitoring and measuring presence of compounds. Such an apparatus and method is need for a variety of applications, such as identifying toxic gas leaks, and measuring ammonia in water and soil samples.
There are a variety of sensing methods and devices now available, including solution-based reagent chemistry for calorimetric reactions as well as compositions of various solid-phase polymer based (e.g., silicone) optical sensors, which includes paintable indicators for detecting ammonia leaks. In addition to being commonly used for measuring acid and base solutions, pH indicator dyes have been used for sensing ammonia (a basic compound). In some cases the pH indicator dye is contained in a formulation, such as common litmus test paper, or in Drager gas-sampling tubes, that only gaseous ammonia can be measured. Such devices either cannot be exposed to liquids, or can only be exposed to liquids for a brief period, or can only be exposed one time. This problem exists because the indicator dye dissociates from a solid-phase carrier and then leaches into the liquid. In some cases the indicator dye is used in a formulation that is not color reversible. In one case an optical-chemical sensor for ammonium ions was made using porous PTFE (polytetrafluoroethylene) membrane, however, it was only described as applicable to monitoring water, designed for waste-water measurement. It was reported (Sellien et al., Anal. Chim. Acta, 269:83-88, 1992) to have the limitation that the indicator was reversible only if the membrane was in contact with an aqueous solution. Therefore, there is a need in the art to improve sensing capabilities for volatile acidic or basic compounds, including and especially ammonia.